chore: import upstream snapshot with attribution

search/median_search2.cpp

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+/**
+ * @file
+ * @brief Given a linked list L[0,....,n] of n numbers, find the middle node.
+ *
+ * @details The technique utilized in this implementation is the ["Floyd's
+ * tortoise and
+ * hare"](https://en.wikipedia.org/wiki/Cycle_detection#Floyd's_tortoise_and_hare)
+ * approach. This technique uses two pointers that iterate through the list at
+ * different 'speeds' in order to solve problems. In this implementation, for
+ * every iteration the slow pointer advances one node while the fast pointer
+ * advances two nodes. The result of this is that since the fast pointer moves
+ * twice as fast as the slow pointer, when the fast pointer reaches the end of
+ * the list the slow pointer will be pointing to the middle node of the list.
+ *
+ * Here are some example lists you can use to see how the algorithm works
+ * A = [1,2,3,4,5]
+ * B = [1,2,3,4,5,6]
+ * print median(A) #should be 39
+ * print median(B) #should be 4
+ *
+ * @author [Benjamin Weiss](https://github.com/weiss-ben)
+ * @see median_search.cpp
+ */
+
+#include <cassert>   /// for assert
+#include <iostream>  /// for IO operations
+
+/**
+ * Definition for singly-linked list.
+ */
+struct ListNode {
+    int val{0};               ///< the value stored in the node
+    ListNode* next{nullptr};  ///< pointer to the next node
+    ListNode() = default;     ///< default constructor
+    explicit ListNode(int x)
+        : val(x) {}  ///< constructor with value for node->val provided
+    ListNode(int x, ListNode* next)
+        : val(x),
+          next(next) {
+    }  ///< constructor with values provided for node->val and node->next
+};
+
+/**
+ * @namespace search
+ * @brief Search algorithms
+ */
+namespace search {
+/**
+ * @namespace median_search
+ * @brief Functions for the Median Search algorithm implementation. Wkipedia
+ * link to algorithm: https://en.wikipedia.org/wiki/Median_search
+ */
+namespace median_search2 {
+/**
+ * This function searches for the median of a linked list.
+ * @param head The head of the linked list.
+ * @returns Median node of the linked list.
+ */
+ListNode* middleNode(ListNode* head) {
+    if (!head) {
+        return nullptr;
+    }
+
+    // Fast and slow pointers
+    ListNode* fastptr = nullptr;
+    ListNode* slowptr = fastptr = head;
+
+    // fast jumps 2 while slow jumps 1
+    while (fastptr->next && fastptr->next->next) {
+        slowptr = slowptr->next;
+        fastptr = fastptr->next->next;
+    }
+
+    return (fastptr->next) ? slowptr->next : slowptr;
+}
+
+void deleteAll(const ListNode* const head) {
+    if (head) {
+        deleteAll(head->next);
+        delete head;
+    }
+}
+}  // namespace median_search2
+}  // namespace search
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test() {
+    auto* head1 = new ListNode;
+    head1->val = 1;
+
+    ListNode* temp = head1;
+    for (int i = 2; i < 6; ++i) {
+        // Allocate next
+        auto* temp1 = new ListNode;
+        temp1->val = i;
+
+        // Advance
+        temp->next = temp1;
+        temp = temp1;
+    }
+    temp->next = nullptr;
+
+    const ListNode* const median = search::median_search2::middleNode(head1);
+    assert(3 == median->val);  // 3 is the value of the median node.
+    search::median_search2::deleteAll(head1);
+    std::cout << "test case:1 passed\n";
+
+    // Test case # 2
+    auto* head2 = new ListNode;
+    head2->val = 1;
+
+    ListNode* temp2 = head2;
+    for (int i = 2; i < 7; ++i) {
+        // Allocate next
+        auto* temp3 = new ListNode;
+        temp3->val = i;
+
+        // Advance
+        temp2->next = temp3;
+        temp2 = temp3;
+    }
+    temp2->next = nullptr;
+
+    const ListNode* const median1 = search::median_search2::middleNode(head2);
+    assert(4 == median1->val);  // 4 is the value of the median node.
+    search::median_search2::deleteAll(head2);
+    std::cout << "test case:2 passed\n";
+
+    std::cout << "--All tests passed--\n";
+}
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main() {
+    test();  // run self-test implementations
+    return 0;
+}